1. Field of the Invention
The present invention relates generally to optical devices and, in particular, to a lamination-based method for the micro-fabrication of optical assemblies and other small components, and to a temperature compensator for a Faraday rotator.
2. Background of the Related Art
When light rays emitted from a light source are transmitted through an optical system, part of the light rays will be reflected at the end face of the optical system and transmitted back to the light source, unless means are employed to prevent such back reflection. For instance, in transmitting an optical signal through an optical fiber, if a light beam emitted from a laser light source is projected onto the end face of the optical fiber through, for example, a lens, the majority of the light thereof will be transmitted through the optical fiber as transmitted light beam. But, a part of the light thereof will be surface reflected at the end faces of the lens and the optical fiber and transmitted back to the laser light source. This back reflected light will again be reflected at the surface of the laser light source, thereby creating undesirable reflection-induced noise.
To eliminate such noise, an optical isolator, as described for instance in Bellcore""s Special Report, Optical Isolators: Reliability Issues, SR-NWT-002855, Issue 1, December 1993, Pages 1-3, incorporated herein by reference, may be used. This is an example of an optical device that allows light to propagate (with relatively low loss) in one direction but isolates reflected light from propagating in the reverse direction. Optical isolators are used to improve the performance of many devices such as external modulators, distributed feedback lasers, Fabry-Perot lasers, semiconductor amplifiers, and diode-pumped solid-state lasers among others.
Optical isolators are typically passive, non-reciprocal optical devices based on the Faraday effect. In 1842, Michael Faraday discovered that the plane of polarized light rotates while transmitting through glass which is contained in a magnetic field. The Faraday effect is non-reciprocal, meaning that the direction of rotation is independent of the direction of light propagation, and only dependent upon the direction of the magnetic field. Most commercial optical isolators utilize this effect to isolate various parts of an optical communication system from reflection-induced noise.
Typically, an optical isolator consists of a magneto-optical material called a Faraday rotator which is sandwiched between a pair of polarization elements commonly referred to as a polarizer and an analyzer. The Faraday rotator is used in optical devices, such as the optical isolator, to rotate the plane of polarization that is incident upon it by a predetermined amount, usually by 45xc2x0 either clockwise or counter clockwise. Typically, the Faraday rotator is a garnet crystalline structure with an inherent magnetic field, so that the direction of Faraday rotation is predetermined. In some cases an external magnetic field may be needed to activate the Faraday rotator. In such cases, the direction of Faraday rotation is dependent on the orientation of the magnetic field but not on the direction of light propagation. As used in the telecommunication industry, the Faraday rotator is essential to many devices that utilize its properties in combination with reciprocal polarization elements.
In the pass (forward) direction, light incident on the polarizer will pass through the polarizer without obstruction if its plane of polarization coincides with that of the polarizer. When this light passes through the Faraday rotator its plane of polarization is rotated by 45xc2x0 due to the magneto-optic effect. The direction of rotation, that is, clockwise or counter clockwise, is dependent on the particular Faraday rotator configuration and is predetermined. The light then passes through the analyzer without loss, since the axis of polarization of the analyzer is oriented at the same 45xc2x0.
In the blocking (reverse) direction, reflected light of arbitrary polarization is incident on the analyzer which transmits some of this light and polarizes it to match its axis of polarization. When this polarized reflected light passes through the Faraday rotator its plane of polarization is again rotated by 45xc2x0, clockwise or counterclockwise relative to the direction of light travel, as is predetermined. As a result, the plane of polarization of the reflected light incident on the polarizer is perpendicular to its axis of polarization, and, thus the reflected light is blocked by the polarizer. In this manner, the optical isolator is used to transmit light from a source in the pass (forward) direction and essentially extinguish any reflected light in the blocking (reverse) direction. This extinguishing effect is commonly known as xe2x80x9cisolationxe2x80x9d.
The magnitude of the rotation of the plane of polarization of light passing through the Faraday rotator depends on several factors, such as, the strength of the magnetic field, the nature of the material that constitutes the rotator, the frequency of the light, the temperature, and other parameters. Since the components in many optical applications utilizing the Faraday effect may be exposed to temperature variations, the rotational temperature dependency of the Faraday rotator limits the use of Faraday rotators in devices which do not provide some form of temperature compensation to prevent or minimize degradation in performance. The rotational temperature dependency of a Faraday rotator can be expressed in terms of a temperature coefficient of rotation, CROT, defined as:       C    ROT    =            ⅆ      θ              ⅆ      T      
where, xcex8 is the rotation of the plane of polarized light passing through the Faraday rotator, and T is the temperature. A typical Faraday rotator may have a temperature coefficient with a magnitude of as much as about 0.10xc2x0/xc2x0 C. which can cause a variation of Faraday rotation of about 12xc2x0 over a temperature range of about xe2x88x9240xc2x0 C. to 85xc2x0 C. Of course, such undesirable rotation of the light can have significant detrimental effects on the performance of an optical device both in terms of forward transmissivity and degree of reverse isolation. But, since isolation (attenuation in the blocking direction) is measured very close to zero, small changes can have orders of magnitude effects on the degree of isolation in terms of the blocking direction transmission of reflected light.
One proposed solution to this problem is to provide temperature compensation via a cooling/heating source which maintains the temperature of the Faraday rotator, and possibly the temperature of the entire device, including for example, the laser source, at the required value. This would require that the temperature of the Faraday rotator be monitored and the output from the cooling/heating source be adjusted accordingly. Thus, the components required in such a temperature compensation system would include a cooling/heating source, temperature measurement device, a feedback system, and a power supply among others. Disadvantageously, such a temperature compensation scheme not only adds to the complexity and cost of the device, but, also to the size of the optical device which can limit the use of the device in many applications.
In some cases, a cascaded isolator, such as a double stage isolator, is utilized to compensate for the effects of temperature variance on optical isolators. Typically, a double stage isolator utilizes a polarizer, a Faraday rotator, an analyzer/polarizer, a second Faraday rotator, and a second analyzer arranged in this sequence. This effectively provides two stages of optical isolators in series. Typically, to compensate for temperature variations, one stage is xe2x80x9cde-tunedxe2x80x9d to an offset temperature above the ambient temperature while the other stage is xe2x80x9cde-tunedxe2x80x9d to an offset temperature correspondingly below the ambient temperature so as to provide a more broad-band response between the two temperature extremes. However, such detuning results in overall degraded isolation performance over the temperature range and at the nominal design temperature. Another proposed solution is to cascade multiple stages of isolators. But, the use of cascaded isolators in an optical device, undesirably, not only adds to the complexity, cost and size of the device, but, also increases the number of components needed, and increases the optical path of the light while reducing overall transmissivity through the cascaded isolators.
Optical components, such as the polarizer, the Faraday rotator, and the analyzer of a typical optical compensator, are commonly fixed in an assembly or attached to a common substrate. The primary approach, in the industry to date, to fixturing optical components involves the implementation of screw-machined barrels or small blocks with counter-bored features machined in. The optical components are placed in these machined cavities which typically tend to be small in size (for example, less than 2 mm in diameter). Not only is the machining process of generally tiny metal fixtures a costly and time consuming operation, but, also the discrete approach of fixing the optical components is generally not suited for mass automation. Undesirably, such a method of fixturing optical components is labor intensive and leads to higher manufacturing costs and lower manufacturing efficiency.
Thus, there is a need for providing a Faraday rotator temperature compensator that is simple, low cost and dimensionally small and there is a need to provide an efficient and low cost method that is well adapted for the automated manufacturing of such optical assemblies and other small components.
A micro-fabrication method in accordance with one preferred embodiment of the present invention overcomes some or all of the afore-mentioned disadvantages. The method employs, in one embodiment, a lamination manufacturing procedure to create an array of optics-receiving micro-fixtures for receiving optical elements to form optical assemblies. An optical isolator is also provided and utilizes the expansion/contraction properties of a bimetallic element to compensate for the effect temperature on Faraday rotation.
The present invention provides thermal compensation in devices utilizing magneto-optical materials, such as Faraday rotators, by utilizing the opening and closing arcing motion of coiled bimetallic metal strips due to their expansion/contraction when exposed to temperature variations. A polarization element is attached to a bimetallic element which allows correction for temperature induced Faraday rotation. The bimetallic element is configured to optimally match the degree of temperature-induced drift in the Faraday rotation with the rotation of the polarization element.
In one preferred embodiment of the present invention a temperature compensated optical isolator is provided. Preferably, the optical isolator includes a pair of polarization elements, a Faraday rotator and a bimetallic element. One of the polarization elements is an optical polarizer while the other polarization element is an optical analyzer. The Faraday rotator is positioned between the polarizer and analyzer. The bimetallic element is attached between the analyzer and a base which houses the polarizer, Faraday rotator, analyzer and bimetallic element.
Advantageously, the bimetallic element of the present invention is configured to optimally conform with the temperature induced changes in Faraday rotation. By rotating the axis of polarization of the analyzer the bimetallic element ensures that any back-reflected light incident on the polarizer has a plane of polarization substantially perpendicular to the polarizer""s axis of polarization. Thus, all or most of the back-reflected light incident on the polarizer will be effectively extinguished, thereby essentially eliminating any temperature-induced degradation in the effective isolation of the optical system.
Advantageously, the bimetallic element of the present invention can be tailored to meet the particular characteristics of the magneto-optical material and is hence adaptable to a wide variety of situations and applications. This is accomplished by appropriate material selection, configuration and dimensioning of the bimetallic element. In one preferred embodiment of the optical isolator of the present invention, the bimetallic element has a generally curved portion which generally circumscribes the analyzer. Of course, other shapes and configurations may also be employed with efficacy, as required or desired, giving due consideration to the goal of optimally enhancing the isolation performance of the optical isolator over a given range of temperatures. Also, the bimetallic element can be used to house a variety of polarization elements and to provide temperature compensation in other optical devices. For example, the bimetallic element may be used in conjunction with a half-wave plate or it may be used in combination with an external modulator, to achieve some or all of the benefits and advantages disclosed herein.
Advantageously, the optical isolator of the present invention provides a significant improvement over conventional single stage isolators by essentially eliminating the effects of temperature on isolation. It is effective in maintaining a consistent optical isolation over an extended temperature range thereby, allowing the optical device to function without costly active temperature control. The isolator of the present invention also provides several advantages over conventional double-stage or cascaded isolators. Desirably, it is lower in cost, simpler in design, dimensionally smaller, is easier to manufacture, and provides a shorter optical path. This simplicity and compactness render the isolator of the present invention a viable choice for providing temperature compensation in a wide variety of optical devices. The size of the isolator allows it to readily fit into standard optical packages. Additionally, the simple construction of the isolator make it a practically effortless retrofit into conventional opto-electronic packages. Also, advantageously, the isolator of the present invention is environmentally stable and is well suited for the present and future in the field of telecommunications.
The present invention also prescribes, in accordance with one embodiment, a preferred micro-fabrication method of manufacturing sub-assemblies of optical elements, such as in one embodiment the temperature compensated optical isolator. Preferably, the method utilizes lamination or layering of metal sheets with arrays of micro-frames to form an array of optic-receiving micro-fixtures. The micro-frames are, preferably, photo-chemically etched or stamped into the sheets and are supported by tab members. Another preferred method employs the molding of engineering plastics to form an array of optic-receiving micro-fixtures which can then be used in substantially the same or similar manner to the metal micro-fixtures. The plastic material can comprise liquid crystal polymer (LCP), polyetheretherketone (PEEK), thermoplastic polyimide (TPI), polyphthalamide (PPA), nylon, teflon and phenolic, among others. Appropriate optical elements are inserted into the micro-fixtures as dictated by the particular application. The laminate units or pallets which accommodate the array of optical elements are stacked, aligned and attached to form an array of optical assemblies in a laminate stack. A base member may be attached to the optical assemblies to facilitate their mounting. The optical assemblies are removed from the laminate stack by conventional trimming methods.
Advantageously, such a method is well suited for the automated manufacturing of optical assemblies and results in high speed, high volume production, thereby desirably maintaining low manufacturing costs. For example, the method can be used in combination with conventional pick-and-place type of robotics. The present method provides an improvement over conventional manufacturing of optical assemblies which typically utilizes a laborious, time-consuming and costly machining process.
Advantageously, the micro-fabrication method can be customized to form or assemble a wide variety of optical components and other small components, and is adaptable to a wide range of applications. For example, the method may be used to mount lenses, crystals, gratings, filters, fibers and various sub-assemblies, among others.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.